Turbo-prop exhaust nozzle control system utilizing impact and exhaust pressures as parameters



2,537,772 SYSTEM UTILIZING As PARAMETERS RD ND A'TTURNEY Patented Jan.9, 1951 TURBO-PROP EXHA UST NOZZLE/ CONTROL SYSTEM UTILIZING IMPACT ANDEX- HAUST PRESSURES AS PARAMETERS Wilton G. Lundquist, Hohokus, andKenneth A.

Browne, Ridgewood, N. J.,

assignors to Wright Aeronautical Corporation, a corporation of New YorkApplication November 30, 1944, Serial No. 565,980

9 Claims. 1

'Ifhis invention relates to an internal combustion engine installationfor a vehicle in which the vehicle driving thrust is derived not onlyfrom the power output of the engine but also from the jet propulsivethrust ofthe engine exhaust. The invention is particularly concernedwith a gas turbine power plant installation for aircraft in which theturbine drives an aircraft propeller and the turbine exhaust alsoprovides an aircraft propulsive thrust. Such a power plant is commonlycalled a turbo-prop.

In a turbo-prop power plant for aircraft. the jet ypropulsive eifect ofthe exhaust gases may be effectively used to help the propeller inproviding the aircraft with forward thrust. In such an installation thetotal thrust power per pound of fuel and air is a variable which dependson the jet discharge velocity of the exhaust gases. For a giveninstallation, aircraft speed and altitude, this total thrust power perpound of fuell and air (thrust efficiency) is a maximum at a particularvalue of the exhaust discharge velocity, and this total thrust powerdecreases if the exhaust dis- .charge velocity increases or decreasesfrom this particular value. Accordingly, it is an object of thisinvention to regulate the exhaust nozzle discharge area to adjust theexhaust discharge velocity for maximum thrust eiliciency. Actually theexhaust discharge velocity for maximum thrust emciency varies with theaircraft velocity and with a function which depends on the turbine andpropeller efficiencies and on the velocity coeflicient of the exhaustnozzle. Therefore it is a further object of this invention to adjust theexhaust nozzle discharge area with variations in the exhaust velocityand velocity of the aircraft.

At this point it should be noted that as herein used, the "impactpressure of a iiuid is equal to the sum of the pressures correspondingto the static and velocity heads of the fluid, that is to the pressurecorresponding to the total head of the uid.

In aircraft having a gas turbine power plant with 4a forwardly directedair opening, the total head or impact pressure of the entering air willvary with the air speed of the aircraft. However, it is desirable thatthe volume flow relations of fluid through the turbine power plant beindependent of the aircraft speed. This result can be obtained by socontrolling the turbine exhaust discharge area to maintain a constantproportion between the total head or impact pressure of the entering airand the total head or impact pressure of the turbine exhaust. 'I'hat is,if the exhaust 2 nozzle discharge area is adjusted to maintain constanttheratio of the total head or impact pressure of the exhaust gases tothe total head or impact pressure of the entering air, then variationsin speed of the aircraft will have substantially no effect on the volumeow relations of iiuid through the turbine power plant. Accordingly it isan object of this invention to maintain a linear or proportionalrelation between the total head or impact pressure of the exhaust gasesand the total head or impact pressure of the entering air.

Applicants have also discovered that by maintaining the impact pressureof the surrounding undisturbed air relative to the aircraft equal to theimpact pressure of the exhaust gases, that the exhaust gas velocity issubstantially equal to its magnitude for maximum thrust eiliciency. In

the absence oi any substantial pressure rise in the intake duct from theaircraft propeller or pressure loss in the intake duct because of theiiow therethrough, the total impact pressure of the surroundingundisturbed air relative to the aircraft is substantially equal to theimpact pressure of the air in the power plant air intake duct.Accordingly it is an object of this invention to adjust the exhaustnozzle discharge area to maintain the impact pressure of the exhaustgases substantially equal to the impact pressure of the surroundingundisturbed air. In this way, the exhaust Velocity is automaticallyadjusted for maxie mum thrust efficiency and in addition the volume flowrelations of the uid through the power plant are substantiallyindependent of variations in speed of the aircraft.

With conventional propeller and turbine efflciencies, if the exhaustnozzle discharge area is adjusted to maintain the impact pressure of theexhaust gases substantially equal tothe impact pressure of thesurrounding undisturbed air relation tothe aircraft, the resultingexhaust velocity will be slightly larger than the value for maximumthrust efficiency. If this discrepancy should be appreciable,substantially maximum thrust efflciency may be obtained by maintaining aratio other than unity between the impact pressure of the exhaust gasesand the. impact pressure of the surrounding undisturbed air relative tothe aircraft. However, an exhaust velocity somewhat larger instead ofsmaller than the value for maximum thrust is desirable since then theturbine power output is less than the power output at maximum thrusteiiiciency and therefore the required size of the turbine iscorrespondingly smaller.

Other objects of this invention will become apparent upon reading theturbine exhaust nozzle discharge area is addusted by the automaticcontrol of the present invention;

Figure 2 is a modification of Figure 1:l Figure 3 is a view of amodified form of control;

Figure 4 is a partial view oi' a further modification.

Referring to Figure 1, a gas turbine powerplant "III for an aircraftcomprises a forwardly opening air intake duct l2 for an air compressorunit I4 having a pair of centrifugal impellers IB. The air delivered bythe compressor unit Il enters the combustion chamber it to which fuel issupplied by nozzles I3 connected to a fuel manifold i9. 20

The combustion gases from the combustion chamber Il are properlydirected into the turbine blades of the turbine rotor 22 by a turbinenozzle 24, and uponleaving the turbine blades the combustion gases aredischarged through the exhaust duct 2 8. The turbine rotor 22 isdrivably connected by a shaft 21 to the compressor unit i4 and thencethrough suitable reduction gearing 28 to an aircraft propeller 29. I Thepropeller 29 is provided with conventional pitch changing mechanism (notshown) for maintaining the propeller and its driving turbine at asubstantially-constant pre-set rotative speed. For example, said pitchchanging mechanism may be similar to that illustrates in Patent No.2,449,452 to c. W. au

Chiilson. With this combination the total aircraft thrust is a sum ofthe propeller thrust de-l rived from the turbine and thejet propulsivathrust of` the exhaust gases. As illustrated the turbineis also drivablyconnected to a drive unit 3| by means of a. turbine shaft 33, the unit3i providing drives for various auxiliary power plant equipment.

The discharge area of the annular exhaust duct 28 and therefore thedischarge velocity of the exhaust gases is controlled by suitableexhaust flaps 30. Reversible electric motors 32 for adjusting theexhaust flaps may be connected thereto through gearing 3l, rack 36 and alink 38. A single motor 32 may be provided for adjusting al1 of theflaps or as illustrated a single motor is provided for adjusting eachindividual flap. Obviously any suitable connection may be providedbetween the motor or motors 32 and flaps 30.

The electric motor or motors 32 are controlled by a flexible diaphragm.l0 which is supported by a housing 42 and extends thereacross to dividethe housing into two chambers 44 and 46. The impact pressure of the 'airentering the intake duct I2 is transmitted to the chamber through aconduit 43 and the impact pressure of the exhaust gases in ,the exhaustduct 26 is transmitted to the chamber IB through a conduit 50. In thisway the control diaphragm I0 is subjected t'o the pressure differentialbetween the entering air impact pressure and the exhaust gas impactpressure.

The housing I2 is provided with switch contacts 52 and 5l which arearranged to be engaged by the diaphragm upon deflection of the diaphragmto the left or right respectively thereby completing circuits to theilap motor or motors 32. Thus, when the diaphragm 40 is deflected to theleft, for example in response to a decrease in the impact pressure ofintake air, the diaphragm 11 is a schematic view of a gas turbine a Il:anar-,Wir

4 engages the switch contact 52 to complete a circuit through the line53 and source' of electric venergy to operate the nap motors 32 in adirection to .effect an opening adJustment of the 5 flaps 30.Similarly,`if the-diaphragm 40 is deilected to the right, for example inresponse to an increase in the impact pressure of the intake air, thediaphragm I0 engages the switch contact I4 to complete a circuit throughthe source of .1 energy 60 and a line 82 to operate the flap motors foreffecting a closing adjustment of the flaps 30.

With this structure, the flap motors are automatically operated inresponse to changes in the aircraft velocity and the exhaust dischargevelocity to maintain the impact pressure of exhaust gases equal to theimpact pressure of the entering air. Accordingly the volume flowrelations of flow through the gas turbine power plant are thensubstantially independent of variations in the velocity of the aircraft.

With the above described aircraft power plant, the total aircraftforward thrust is equal to the 25 sum of the propeller thrust and thejet propulsive effect of the exhaust gases. At a constant rotative speedof the turbine, the total aircraft thrust power per pound of fuel andair has a maximum or peak value for a particular discharge 30 velocityofthe exhaust gases. The particular exhaust velocity at which thismaximum or peak thrust occurs depends not only on the aircraft speed andaltitude. but also on the efficiencies of the air compressor and turbineand on the velocity coefficient of the exhaust nozzle. If the impactpressure of the exhaust gases is maintained substantially equal to theimpact pressure of the surrounding undisturbed air relative to theaircraft, the exhaustdischarge velocity will substantially correspond tothe value for maximum thrust efliciency. As illustrated, the intake ductI2 opens forwardly about the root ends of the propeller blades andtherefore the impact pressure of the air in the intake duct will b esubstantially equal to that of the surrrounding undisturbed air relativeto the aircraft. Accordingly the aforedescribed control of the exhaustflaps 30 by maintaining the impact pressure head of the exhaust gasessubstantially equal to the impact pressure head of the entering air, notonly renders the volume flow relations of the fluid through the turbinepower plant independent of the aircraft speed, but also maintains theexhaust discharge velocity at the proper magnitude for maximum thrustefllciency.

Since the total head or impact pressure of the air entering the intakeduct is substantially equal to the .total head or impact pressure of thesurrounding undisturbed air stream relative to the so aircraft, theexhaust flaps 3|! may be regulated by subjecting one side of thediaphragm l0 to the impact pressure measured in this surrounding airinstead of measured in the air entering intake duct I2. Figure 2partially illustrates such o5 a modification. The modification of Figure2 is similar to Figure 1 except a Pitot tube 48' has been substitutedfor the tube 4B of Figure 1, the Pitot tube 48 extending out into thesur- `rounding undisturbed air stream about the air,-

craft. The arrangement of Figure l has the advantage in that the tube 48does not project into the airstream about the aircraft and is selfcontained within the power plant. Even if the turbine power plant doesnot have a forwardly I5 directed air intake opening, the flap control of.muren merma may be used for maximum thrust eiliciency.

At this point it should be noted that, although the invention has beendescribed` in connection turbine type power pl nt, the invenalso isapplicable to other internal combustion engines, for example aconventional reciprocating internal combustion engine.

As previously noted, in general if the impact pressure of the exhaustgasesismaintained equal to the impact pressure of the undisturbedvsurrounding air relative to the aircraft, the resulting exhaust velocityis slightly larger thanthe value for maximum thrust efficiency. However,this result depends on the efficiencies of the compresser and turbineand on the velocity coemcient of the exhaust nozzle. For -this reasonand because the propeller hub may add slightly to the pressure of theair entering the intake duct it may be desirable to maintain the ratioof these impact pressures something other than unity. for example asillustrated in Figure 3, in order to maintain a desired thrustefficiency. In *bis modification a housing 1li is divided into a pair ofend chambers 12 and 14 and an intermediate chamber 18 by a pair ofdiaphragms 18 and 80 of unequal area. The intermediate chamber 16 isvented to the static pressure of the surrounding atmosphere throughpassage 82 while end chambers 12 and 1l are subjected to the impactpressure of the entering air and exhaust gases respectively throughconduits 84 and 86. Instead of measuring the impact pressure of theentering air in the intake duct I2, the conduit B4 may, as illustrated,extend out into the surrounding airstream in the form of a Pitot tubesimilar t tube 48 in Figure 2. The connected together for joint movementby a stud 88. A pair of switch contacts 90 and 92 are arranged in amanner similar to the switch contacts in Figures 1 and 2, such that upondeflection of the diaphragm assembly to the left, contact 90 is engagedto complete a circuit to the flap motors to effect an opening adjustmentof the flaps. Similarly, upon movement of the diaphragm assembly to theright, contact 92 is engaged to complete a circuit to the flap motors toeffect a closing adjustment of the flaps.

With the structure of Figure 3 so far described and because thediaphragms 18 and 80 are of unequal area, the exhaust flaps will beautomatically adjusted to maintain an exhaust gas impact pressure whichis less than and proportional to the entering air impact pressure.During engine operation with the aircraft parked on the ground and withthe propeller having little or no eiect on the intake air pressure induct I2, the impact' pressure within the intake duct I2 is substantiallyequal to atmospheric pressure and therefore the impact pressure of theexhaust gases necessarily is larger and the diaphragms 18, 8U Awilldeflect to the left to fully open the exhaust flaps III. Similarly, inFigure 1 when the engine is operated with the aircraft on the ground,the impact pressure of the exhaust gases will be effective to urgediaphragm I0 to the left to fully open the exhaust ilaps. However, it isdesirable that the discharge velocity of the exhaust gases be maintainedabove a predetermined value. This result may be accomplished bydisposing a spring so as to urge the control diaphragm against the forceproduced by the impact pressure of the exhaust gases thereon. Such aspring is illustrated at 94 in Figure 3. Accordingly the spring $4 isdesigned to exert a force such that, while the aircraft is on theground, the magnitude of the impact pressureof the exhaust gases againstdiaphragm lli required to balance this.- spring force will result in thedesired minimum exhaust gas discharge velocity. a similar spring may beprovided in ure 4 is identical with Figures 1 and 2 except for theaddition of the spring 94 so that no further description of Figure 4appears necessary. The

` addition of the spring 94 to Figure 3, as shown in diaphragms 18 and80 are standing our invention,

Figure 4, also makes it possible to maintain the impact pressure of theintakeair substantially equal to the impact pressure of the exhaustgases through at least a limited operating range in addition to theprovision of a predetermined minimum exhaust discharge velocity. Tothisend. the diaphragm 1B is made sufficiently smaller than thediaphragm so that during a limited operating range, the spring force isapproximately equal to the force exerted by the exhaust gas impactpressure on that much of the area of diaphragm 80 in excess of the areaof diaphragm 18, and therefore, at least in said limited range, theimpact pressure of the exhaust gases will be maintained substantiallyequal to the impact pressure of the entering air.

While we have described our invention in detail in its present preferredembodiment, it will bevobvious to those skilled in the art, afterunderthat various changes and modifications may be made therein withoutdeparting from the spirit or scope thereof. aim in the appended claimsto cover all such modifications.

We claim as our invention:

l. A vehicle gas turbine power-plant having a combustion chamber; aturbine arranged t0 be driven by combustion gases from said chamber; acompressor drivably connected to said turbine for supplying said chamberwith compressed air; vehicle propelling means also drivably connected tosaid turbine; a forwardly directed air intake duct for said compressor;a rearwardly directed exhaust duct through which the turbine exhaustgases continuously discharge during turbine operation; an adjustabledischarge nozzle for said exhaust duct; means providing a rst forcewhich is substantially proportional to the impact pressure of thesurrounding air relative to said power plant; means providing a -secondforce which is substantially proportional to the impact pressure of theexhaust gases discharging through said duct; means responsive to changesin said forces; and means controlled by said responsive means foradjusting said nozzle so that an increase in said first-mentioned impactpressure and a decrease in said second-mentioned impact pressure bothtend to effect a closing adjustment of said nozzle.

2. A vehicle gas turbine power plant having a combustion chamber; aturbine arranged to be driven by combustion gases from said chamber; acompressor drivably connected to said turbine for supplying said chamberwith compressed air; vehicle propelling means also drivably connected tosaid turbine; a forwardly directed air intake duct for said compressor;a rearwardly directed exhaust duct through which the turbine exhaustgases continuously discharge during turbine operation; an adjustabledischarge nozzle for said exhaust duct; means providing a first forcewhich is substantially proportional to the impact pressure of thesurrounding air relative to said power plant; means providing a secondforce which is We.I

exhaust duct through f v7 bstantlally proportional su or theexhaustgases discharging through said I impact pressures substantially equal.

3. A vehicle gas turbine A) ment of said nozzle.

4. A vehicle gas turbine power combustion chamber; a turbine arranged tobe movable means for adjusting said nozzle so as to maintain said impactpressures substantially equal.

`5. A vehicle gas turbine power plant having a combustion chamber; aturbine arranged to be driven by combustion -gases from said chamber; acompressor drivably connected to said turbine for supplying said chamberwith compressed air; vehicle propelling means also drivably connected tosaid turbine; a forwardly directed air intake ductfor said compressor; arearwardly directed which the turbine exhaust gases continuouslydischarge during turbine ope eration; an adjustable discharge nozzle forsaid exhaust duct; a housing having a. pair of chambers; means forsubjecting one of said chambers to a iiuid pressure substantiallyproportional to the impact pressure of the surrounding air relative tosaid power plant; means for subjecting the other of said chambers to afluid pressure substantially proportional to the impact pressure of theexhaust gases discharging through said exhaust duct; movable meansseparating said chambers and movable in response to changes in saidiiuid pressures; and means automatically controlled only by said movablemeans for adjusting said nozzle so that an increase in saidfirst-mentioned uid pressure and a decrease in id second-mentioned fluid7. An aircraft gas turbine power plant having a combustion chamber, aturbine arranged to be driven by combustion gases from said chamber, acompressor drivably connected to said turbine chamber with compressedair,

first-mentioned iluid pressure and a decrease in said second-mentionediluid pressure both tend to eilect a closing adjustment of said nozzle.

8. An aircraft gas turbine power plant having a combustion chamber, aturbine arranged to be driven by combustion gases from said chamber, acompressor drivably connected to said turbine for supplying said chamberwith compressed air, a bladed aircraft propeller also drivably connectedto said turbine, a forwardly directed air intake duct for saidcompressor, a, rearwardly directed exhaust duct through which theturbine exhaust gases continuously discharge during turbine operation,and an adjustable discharge nozzle for said exhaust duct; meanscommunicating with said intake duct and providing a, fluid pressuresubstantially proportional to the impact pressure of the air flowingthrough said duct; communicating with said exhaust duct-and providing ailuid pressure substantially proportional to the impact pressure of theexhaust gases discharging therethrough; means responsive to changes insaid fluid pressures; and means controlled by said responsive means foradjusting said nozzle so that an increase in said first-mentioned fluidpressure and a decrease in said second-mentioned uid pressure both tendto eiect a closing adjustment of said nozzle. 9. In combination with avehicle; a gas turbine power plant having a combustion chamber; ,creasein said discharge velocity both tend to a turbine arranged to be drivenby combustion effect operation of said adjustable means in a gases fromsaid chamber; a compressor drivably direction to decrease said dischargevelocity. connected to said turbine for supplying said mm chamber withcompressed air; vehicle propelling 5 AQ'SO'IQNE means also drivablyconnected to said turbine;

a forwardly directed air intake duct for said com- REFERENCES CITEDpressor; a rearwardly directed exhaust duct through which the turbineexhaust gases comin- The following references are of record in theuously discharge during turbine operation; means o file 0f this4pllieni'f adjustable to vary the discharge Vvelocity of the exhaustgases; means responsive to changes in UNITED STATES PATENTS' the airvelocity of the vehicle and to changes in Number Name Date the dischargevelocity of the exhaust gases; and 2,280,835 LyShOlm Apr. 28, 1942 meanscontrolled by said responsive means for 15 2,326,072 Seippel Aug. 3,1943 effecting operation of said adjustable means so 2,342,262 Franz etal Feb. 22, 1944 that a decrease in said air velocity and an in-2,411,895 Poole Dec. 3, 1946

